What is the Loch Ness Monster? No one knows, but that hasn't stopped legions of armchair cryptozoologists from formulating one theory after another on the subject of the world's most famous lake creature.

Ok, it isn't entirely true that we don't know anything about what Nessie is – a lot of evidence indicates that the beast was born of the marriage between a compelling local legend and the imaginations of hoaxers and excitable eyewitnesses alike. The cold, murky waters of that massive lake must surely take on an ominous, mysterious quality during long Scottish nights – is it any surprise that the region generated the Greatest Fish Story Ever Told? Since the first publicized sighting in 1933, the adventurous and the curious have been trying to figure out what it is that people keep seeing in those turbid grey waters.

Theory One: Nessie is a Plesiosaur.
This theory might not seem weird because it's been around almost since the original sighting. There are even a few oddly plausible aspects of this theory. Plesiosaurs (specifically, long-necked elasmosaurs) may have been warm-blooded, which would allow one to live in the chilly loch waters. In the early 90s, a Discovery Channel expedition learned that the loch's fish population was much greater than previously known – enough to support a population of evolved plesiosaurs? Maybe.
There are two huge problems with this theory, though. The biological problem is that elasmosaurs were not physically able to raise their heads and necks above the water in the swan-like fashion virtually every photo and eyewitness account indicates. The geological problem is more severe: in between the supposed extinction of plesiosaurs and the formation of Loch Ness was a period of glaciation that left the entire region encased in ice several miles thick. And if you're about to propose some kind of Encino Man scenario, let me just stop you before you say it out loud. No, just stop.

Theory Two: Nessie is an elephant.
All of the photos of Nessie appear to show objects bearing a vague resemblance to a Loch Ness Monsterish shape, but all of them could easily be something else. There are a lot of plausible theories, like native otters or elephant seals that have been known to venture into the loch from time to time. A few scientists have proposed that the head and neck shape is actually the trunk of an elephant held aloft, with the elephant's back forming the humps of Nessie's…uh, humps. Whether the elephant photos were taken elsewhere and claimed to be from Loch Ness, or some circus elephants escaped and took a swim in a frigid Scottish loch is anyone's guess. But that's cryptozoology for you, where "swimming elephant" can seem perfectly logical.

Theory Three: Nessie is a standing wave from a boat wake.
Loch Ness is really long and narrow, plus extremely deep with sheer sides. This causes waves to do weird things sometimes. If a boat heads down the loch's center, the wake hits the sides and returns to the center to form a standing wave long after the boat has left. A lot of sightings are simply weird wakes in the middle of the loch with no wind or boats nearby, and the standing wave theory would account for these. It's a weird theory because it's so prosaic. I kind of love the idea that all this fuss is over a simple matter of fluid dynamics.

Theory Four: Nessie is tree gas.
This is my personal favorite. The idea is that the loch is surrounded by huge stands of pine trees which fall into the loch when they die. Of course, maybe they just float, and the branches sticking up look like Nessie. Fine. You can even argue that nearby lochs with pine trees have their own monster stories, while non-piney lochs will brook no such nonsense.

But we can create a much weirder theory than that. Suppose some of these logs are covered in sticky pine resin. As they decompose, they fill with gas, but the resin keeps the gases sealed inside. At some point, though, they decompose too much and the seal breaks. All that stored up, pressurized gas jets out one end of the log, propelling the log (and its Nessie-headed branches) along the surface at some speed, creating a wake and, to some witnesses, a terrifying monster. Rocket log!

Theory Five: Nessie is dead.
There are two parts to this theory, one hilarious, one tragic. In part one, a team of Yorkshire zoologists head to the loch in 1972 on a monster-hunting adventure. Locals soon direct them to a massive carcass floating in the water, which they retrieve and load into a van for further research. Alarming descriptions of the beast emerge (a bear's head, scales, claws). However, police stop the van and confiscate the corpse under a law forbidding removal of unknown creatures from Loch Ness. The stunning find is ameliorated when one of the zoologist's colleagues admits to dumping the intentionally disfigured corpse of a bull seal into the loch to fool them, never expecting the police to get involved.

In part two, Robert Rines spends several decades of his life and no small amount of money hunting the Loch Ness Monster. After seeing the beast in person in 1971, all he got for years of effort was a variety of odd sonar contacts. When even those stop showing up in the mid-2000s, Rines finally gives up. But despite the mountain of evidence against the existence of Nessie, and the fact that he himself was unable to find anything in the loch, he refuses to give up his belief in the creature's existence. Rines simply claims that it had become extinct due to global warming.

An honorable mention must go to the "Surgeon's Photo" hoax. You can see the Surgeon's Photo in black and white above. It was a major piece of evidence for years, until one of the perpetrators revealed that it was a toy submarine with a sculpted monster head on top. The best part is that the whole thing was done at the urging of a big-game hunter/adventurer named Marmaduke Wetherell, who had been humiliated in the newspapers a few years earlier by fake Nessie tracks made with a rhino foot umbrella stand. The hoaxed photo was to be his revenge. I'm surprised that they weren't foiled by a group of teenagers and their dog traveling around in a brightly colored van.

This summer, io9 is going cryptozoological. We're offering a $2000 bounty to the person who sends us the best authentic photo or video of a "cryptid," or mystery animal. And that's just the beginning of Cryptid Summer. More »